stuck CTRL key and scrolling the mouse will shrink all text on the screen. so if it happens again hit control and scroll the mouse wheel towards you.
if the text goes back to its original size you likely need a new keyboard.
if both mouse and keyboard are wireless they may conflict at a driver level. very unlikely but possible.
if there wired they can still have conflicts but again rare. check the manufacturers site if the keyboard/mouse use specific drivers for updates or even firmware updates .
worst of all your motherboard may have a problem on the ps2 and mouse controller but you would likely see more keys affected... so i hope its the stuck ctrl key.